5.3: Lipids - Biology


Lipids include oils, fats, and waxes. The body digests lipids by breaking them down into tiny components by emulsification with bile salts and by enzymes called lipases. The emulsification of lipids occurs in the small intestine.

Emulsification of lipids


  • test tubes and rack
  • vegetable oil
  • water


  1. Draw a line at the 1 cm mark and the 2 cm mark on 2 test tubes.
  2. Add vegetable oil to the 1 cm mark in both test tubes.
  3. Add an additional cm of water to each test tube.
  4. Add 10 drops of detergent to one of the tubes
  5. Cover the tubes with parafilm and shake vigorously about 30 times (instructor will demonstrate).


  1. Describe the results (do not include the presence of foam) with respect to lipid

  2. Identify the experimental variable in the lipid experiment.

  3. Which tube represents the control group in the lipid experiment?

  4. List 3 controlled variables in this experiment

  5. What is the dependent variable in the experiment?



Lipid: is one of a class of water-avoiding compounds.
Hydrophobic: avoids water molecules.
Fat: is an organic compound consisting of a three-carbon back-bone (glycerol) attached to three fatty acids (: contains long hydrocarbon chains).
Saturated fat: is fat in which all three fatty acid chains contain he maximum possible number of hydrogen atoms.
Unsaturated fat: is fat with less than the maximum number of hydrogen in one or more of its fatty acid chains. This is because some of its carbon atoms are double-bonded to each other.
Steroid: is a lipid molecule with four fused carbon rings.
Cholesterol: is a steroid molecule present in the plasma membranes of animal and human cells. It can be the starting point from which your body produces other steroids.

Styrene Maleic Acid (SMA)

SMA copolymer is formed from polymerization of a mixture of styrene and maleic anhydride in various ratios (3:1 and 2:1 being the most common). The anhydride moieties can subsequently be hydrolyzed to maleic acid (Figure (PageIndex<2>)). The alternating hydrophobic residues (styrene) and hydrophilic (maleic acid) is thought to be determining for the SMA properties of membrane solubilization.

Figure (PageIndex<2>): Polymerization overview. (CC BY-NC Ümit Kaya)

SMA can be purchased from a range of vendors hereunder Cray Valley, Polyscope Polymers (Xiran) and Sigma-Aldrich (3:1 SMA, Lipodisq).

Microscopy tools for the investigation of intracellular lipid storage and dynamics

Background: Excess storage of lipids in ectopic tissues, such as skeletal muscle, liver, and heart, seems to associate closely with metabolic abnormalities and cardiac disease. Intracellular lipid storage occurs in lipid droplets, which have gained attention as active organelles in cellular metabolism. Recent developments in high-resolution microscopy and microscopic spectroscopy have opened up new avenues to examine the physiology and biochemistry of intracellular lipids.

Scope of review: The aim of this review is to give an overview of recent technical advances in microscopy, and its application for the visualization, identification, and quantification of intracellular lipids, with special focus to lipid droplets. In addition, we attempt to summarize the probes currently available for the visualization of lipids.

Major conclusions: The continuous development of lipid probes in combination with the rapid development of microscopic techniques can provide new insights in the role and dynamics of intracellular lipids. Moreover, in situ identification of intracellular lipids is now possible and promises to add a new dimensionality to analysis of lipid biochemistry, and its relation to (patho)physiology.

Keywords: BODIPY, Boron-dipyrromethene CARS, coherent anti-stokes Raman scattering CLEM, correlative light electron microscopy CLSM, confocal laser scanning microscopy DIC, differential interference microscopy FA, fatty acid FIB-SEM, focused ion beam scanning electron microscopy FLIP, fluorescence loss in photobleaching FRAP, fluorescent recovery after photobleaching FRET, fluorescence resonance energy transfer Fluorescent lipid probes GFP, green fluorescent protein HCV, hepatitis C virus LD, lipid droplet Lipid droplets Live-cell imaging Metabolic disease NBD, nitro-benzoxadiazolyl PALM, photoactivation localization microscopy SBEM, serial block face scanning electron microscopy SIMS, Secondary Ion Mass Spectrometry SRS, Stimulated Raman Scattering STED, stimulated emission depletion STORM, stochastic optical reconstruction microscopy Super-resolution TAG, triacylglycerol TEM, transmission electron microscopy TOF-SIMS, time-of-flight SIMS TPLSM, two-photon laser scanning microscopy Vibrational microscopy.

5.3: Lipids - Biology

Characteristics of Lipids

  • Fats consists of three carbon backbone called Glycerol attached to three fatty acids, which contains hydrocarbon chains.

  • Saturated fat is fat which all three fatty acid chains contain the maximum number of hydrogen atoms.
  • A lipid molecule in which carbon skeletons form four fused rings is called a steriod.
  • Cholesterol is an essential molecule found in membranes that surround your cells.

Concept Checks

1.) What property do lipids share?

Lipids are "water-avoiding" molecules. (Hydrophobic)

2.)What are the parts of a fat molecule?

Fat consists of three carbon backbones called glycerol, which are connected to three fatty acids.

3.)Describe two ways that steroids differ from fats.

Steroids are differnt from fats because of there structure and function.

4.) What does the term unsaturated fat on a food label mean?

Foods labeled unsaturated fat could mean that the product has less than the maximum number of hydrogen atoms. These food are healthier than product with saturated fat.

Testing the hypothesis

To test for the existence of crystalline membrane structures as a consequence of physiological stimuli, it may be possible to use X-ray diffraction. Using small-angle X-ray diffraction, cholesterol crystallites in cell membranes have recently been revealed in atherosclerotic smooth muscle cells, as well as in healthy cells of the human ocular lens, where these structures contribute to the organ's transparency [8]. Using X-ray diffraction to detect crystalline molecular arrangements that would arise transiently and focally in an isolated cell as a result of an extra-cellular stimulus may well, however, be beyond the reach of the currently available technology, and pulse EPR spin labelling or single molecule fluorescence microscopy may represent more feasible approaches to detect very small numbers of molecules switching to a solidified state [9].

As an alternative, an even more direct way would rely on direct microscopic observations. If the model proposed here is true, one could predict that the focal stimulation of a cell would result in the formation of an area preferentially enriched in components know to be recruited in microdomains, whereas remote areas in that same cell would not. This is in fact exactly what happens at the level of an immunological synapse, but the molecular mechanisms involved are far too numerous and complex to be amenable to simple experimental testing. One alternative may be to deliver a very focal stimulatory signal to a cell via the tip of a micro-injection pipet. This could for example be the release of a growth hormone for which the cell's receptor has been described to localize to rafts after stimulation. The prediction from the dock seeding model would be that molecules commonly used as raft markers such as some fluorescent lipids should concentrate around the point of the stimulus delivery. It may be that even GM1 labelled with fluorescent cholera toxin (CT) could be used for such an experiment, but one would have to bear in mind that CT has per se aggregating properties. If concentration of fluorescence around the area of stimulation is observed, it will be necessary to rule out the possibility of a simple coalescence of pre-existing rafts that were initially too small to detect by optical microscopy. For this, one could call upon the fluorescence resonant energy transfer (FRET) technology. If raft coalescence is occurring, the effective surface concentration between raft components will remain unchanged, and FRET signals between those components should remain comparable to those obtained on the same cell at a distance from the focus of stimulation. On the other hand, if areas of solid state form as a consequence of stimulatory signals, FRET signals between these docked components should be significantly enhanced around the focus of stimulus delivery compared to the rest of the cell. A major hurdle for the validity of such an approach, however, will be to identify fluorescent probes that could be incorporated within the assembled crystalline structures without disrupting them.

5.3: Lipids - Biology

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Digestion and Absorption

Digestion is the mechanical and chemical breakdown of food into small organic fragments. It is important to breakdown macromolecules into smaller fragments that are of suitable size for absorption across the digestive epithelium. Large, complex molecules of proteins, polysaccharides, and lipids must be reduced to simpler particles such as simple sugar before they can be absorbed by the digestive epithelial cells. Different organs play specific roles in the digestive process. The human diet needs carbohydrates, protein, and fat, as well as vitamins and inorganic components for nutritional balance. How each of these components is digested is discussed in the following sections.


The digestion of carbohydrates begins in the mouth. The salivary enzyme amylase begins the breakdown of food starches into maltose, a disaccharide. As the bolus of food travels through the esophagus to the stomach, no significant digestion of carbohydrates takes place. The esophagus produces no digestive enzymes but does produce mucous for lubrication. The acidic environment in the stomach stops the action of the amylase enzyme.

The next step of carbohydrate digestion takes place in the duodenum. Recall that the chyme from the stomach enters the duodenum and mixes with the digestive secretion from the pancreas, liver, and gallbladder. Pancreatic juices also contain amylase, which continues the breakdown of starch and glycogen into maltose, a disaccharide. The disaccharides are broken down into monosaccharides by enzymes called maltases, sucrases, and lactases, which are also present in the brush border of the small intestinal wall. Maltase breaks down maltose into glucose. Other disaccharides, such as sucrose and lactose are broken down by sucrase and lactase, respectively. Sucrase breaks down sucrose (or &ldquotable sugar&rdquo) into glucose and fructose, and lactase breaks down lactose (or &ldquomilk sugar&rdquo) into glucose and galactose. The monosaccharides (glucose) thus produced are absorbed and then can be used in metabolic pathways to harness energy. The monosaccharides are transported across the intestinal epithelium into the bloodstream to be transported to the different cells in the body.

Figure (PageIndex<1>): Digestion of carbohydrates is performed by several enzymes. Starch and glycogen are broken down into glucose by amylase and maltase. Sucrose (table sugar) and lactose (milk sugar) are broken down by sucrase and lactase, respectively. Image from OpenStax Biology 2e / CC BY 4.0

Digestion of Carbohydrates

Enzyme Produced By Site of Action Substrate Acting On End Products
Salivary amylase Salivary glands Mouth Polysaccharides (Starch) Disaccharides (maltose), oligosaccharides
Pancreatic amylase Pancreas Small intestine Polysaccharides (starch) Disaccharides (maltose), monosaccharides
Oligosaccharidases Lining of the intestine brush border membrane Small intestine Disaccharides Monosaccharides (e.g., glucose, fructose, galactose)


A large part of protein digestion takes place in the stomach. The enzyme pepsin plays an important role in the digestion of proteins by breaking down the intact protein to peptides, which are short chains of four to nine amino acids. In the duodenum, other enzymes&mdashtrypsin, elastase, and chymotrypsin&mdashact on the peptides reducing them to smaller peptides. Trypsin elastase, carboxypeptidase, and chymotrypsin are produced by the pancreas and released into the duodenum where they act on the chyme. Further breakdown of peptides to single amino acids is aided by enzymes called peptidases (those that breakdown peptides). Specifically, carboxypeptidase, dipeptidase, and aminopeptidase play important roles in reducing the peptides to free amino acids. The amino acids are absorbed into the bloodstream through the small intestines.

Figure (PageIndex<1>): Protein digestion is a multistep process that begins in the stomach and continues through the intestines. Image from OpenStax Biology 2e / CC BY 4.0


Lipid digestion begins in the stomach with the aid of lingual lipase and gastric lipase. However, the bulk of lipid digestion occurs in the small intestine due to pancreatic lipase. When chyme enters the duodenum, the hormonal responses trigger the release of bile, which is produced in the liver and stored in the gallbladder. Bile aids in the digestion of lipids, primarily triglycerides by emulsification. Emulsification is a process in which large lipid globules are broken down into several small lipid globules. These small globules are more widely distributed in the chyme rather than forming large aggregates. Lipids are hydrophobic substances: in the presence of water, they will aggregate to form globules to minimize exposure to water. Bile contains bile salts, which are amphipathic, meaning they contain hydrophobic and hydrophilic parts. Thus, the bile salts hydrophilic side can interface with water on one side and the hydrophobic side interfaces with lipids on the other. By doing so, bile salts emulsify large lipid globules into small lipid globules.

Why is emulsification important for digestion of lipids? Pancreatic juices contain enzymes called lipases (enzymes that breakdown lipids). If the lipid in the chyme aggregates into large globules, very little surface area of the lipids is available for the lipases to act on, leaving lipid digestion incomplete. By forming an emulsion, bile salts increase the available surface area of the lipids many fold. The pancreatic lipases can then act on the lipids more efficiently and digest them. Lipases breakdown the lipids into fatty acids and glycerides. These molecules can pass through the plasma membrane of the cell and enter the epithelial cells of the intestinal lining. The bile salts surround long-chain fatty acids and monoglycerides forming tiny spheres called micelles. The micelles move into the brush border of the small intestine absorptive cells where the long-chain fatty acids and monoglycerides diffuse out of the micelles into the absorptive cells leaving the micelles behind in the chyme. The long-chain fatty acids and monoglycerides recombine in the absorptive cells to form triglycerides, which aggregate into globules and become coated with proteins. These large spheres are called chylomicrons. Chylomicrons contain triglycerides, cholesterol, and other lipids and have proteins on their surface. The surface is also composed of the hydrophilic phosphate "heads" of phospholipids. Together, they enable the chylomicron to move in an aqueous environment without exposing the lipids to water. Chylomicrons leave the absorptive cells via exocytosis. Chylomicrons enter the lymphatic vessels, and then enter the blood in the subclavian vein.

Figure (PageIndex<1>): Lipids are digested and absorbed in the small intestine. Image from OpenStax Biology 2e / CC BY 4.0


Vitamins can be either water-soluble or lipid-soluble. Fat soluble vitamins are absorbed in the same manner as lipids. It is important to consume some amount of dietary lipid to aid the absorption of lipid-soluble vitamins. Water-soluble vitamins can be directly absorbed into the bloodstream from the intestine.


Minerals travel through the digestive tract and into the bloodstream in various ways. For example, potassium is quickly absorbed through the wall of the small intestine and into the bloodstream. It circulates freely throughout the body, and excess is filtered out of the blood and excreted by the kidneys. Some minerals are not absorbed as easily. Vitamin B12 must bind with intrinsic factor, a protein made by cells in the stomach lining, before it can be recognized and absorbed from the intestine into the bloodstream.


Water is primarily absorbed in the small intestine, especially from the first segments of the small intestine - the duodenum and jejunum. It is absorbed easily into the bloodstream. A smaller amount of water is absorbed from the large intestine.

Figure (PageIndex<1>): Mechanical and chemical digestion of food takes place in many steps, beginning in the mouth and ending in the rectum. Image from OpenStax Biology 2e / CC BY 4.0


The final step in digestion is the elimination of undigested food content and waste products. The undigested food material enters the colon, where most of the remaining water is reabsorbed. Recall that the colon is also home to the microflora called &ldquointestinal flora&rdquo that aid in the digestion process. The semi-solid waste is moved through the colon by peristaltic movements of the muscle and is stored in the rectum. As the rectum expands in response to storage of fecal matter, it triggers the neural signals required to set up the urge to eliminate. The solid waste is eliminated through the anus using peristaltic movements of the rectum.

Watch the video: Bio (January 2022).